Field of the Invention
The present invention relates to a method for aligning structures on the front side of a substrate and on the rear side of the substrate.
In the three-dimensional integration of integrated circuits, a thinned semiconductor substrate is arranged on a second semiconductor substrate and is mechanically and electrically connected thereto. This method is described, for example, in xe2x80x9cVertically Integrated Circuitsxe2x80x94A Key Technology for Future High Performance Systemsxe2x80x9d, M. Engelhardt et. al. from CIP ""97 Proceedings, Proc. 11th International Colloquium on Plasma Processes, page 187 (1997), and Supplxc3xa9ment à la Revue Le Vide: science, technique et applications; No 284, April-May-June 1997, Editor: Socixc3xa9txc3xa9 Francaise du Vide, 19 rue du Renard, 75004 Paris, France, on pages 187 to 192 (Supplementary Article to Revue Le Vide: Science, Techniques and Applications). In the method, the alignment and contact-connection of the semiconductor substrate is one of the technologically most demanding and most difficult process steps.
For the three-dimensional integration, it is customary first to provide two wafers having circuit sections that have already been processed. In this case, the first wafer serves as a carrier and the second wafer is thinned by the following method and is arranged on the first wafer. For thinning, first the front side of the second wafer is provided with an adhesive layer, and is connected to a mounting carrier. The front side is the side having the electrical circuits. The second wafer is then thinned from its rear side. In addition, contact hole openings are formed through the thinned substrate from the rear side as far as the first metal plane on the front side of the thinned wafer. Aligning the contact holes with respect to the contacts on the front side of the thinned substrate cannot be effected using the front side of the thinned substrate since the front side is concealed by the mounting carrier.
Therefore, the rear side of the second wafer is conventionally exposed using contact lithography. The alignment is effected using infrared exposure on marks that are arranged on the front side of the thinned wafer. In this case, the contact exposure, and particularly the infrared contact exposure, causes a large alignment error of typically +/xe2x88x925 xcexcm. Therefore, the contact regions on the front side of the thinned substrate are usually made very large. This means that valuable space on the front side of the substrate, which contains the electrical circuits, for example, is occupied by the large contacts.
It is accordingly an object of the invention to provide a method for aligning structures on a substrate front side and a substrate rear side which overcomes the above-mentioned disadvantages of the prior art methods of this general type.
In particular, it is an object of the invention to provide a method for aligning structures on a substrate front side and a substrate rear side which enables a significantly smaller alignment error.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for aligning structures on the front side of a substrate and on the rear side of the substrate, which is opposite the front side of the substrate. The method includes steps of: forming a structure on the front side of the substrate; overgrowing the structure with a useful layer; and uncovering the structure, proceeding from the rear side of the substrate.
The method includes first forming a structure on the front side of the substrate. Afterward, by way of example, further layers can be formed on the front side of the substrate. By way of example, electrical components and circuits are formed in the further layers. In this case, the structure on the front side of the substrate can be used for the alignment in a stepper, which carries out the exposure of a photoresist on the substrate front side. By way of example, by etching the rear side of the substrate, the structure which is used for patterning the substrate front side is uncovered on the substrate rear side, so that it can also be used on the substrate rear side as an alignment mark for a stepper in order to expose a photoresist on the substrate rear side for the processing of the substrate rear side. Since the structure is suitable for being used by a stepper, a contact exposure, which has a high alignment error, is no longer necessary. Equally, it is possible to dispense with the infrared exposure through the thinned wafer, which likewise reduces the alignment error.
One method step includes an alignment step using the structure uncovered on the rear side as an alignment mark. This advantageously enables the use of a stepper exposure instead of an infrared contact exposure, as a result of which the alignment error can be significantly reduced.
A further method step includes an alignment step using the structure on the front side of the substrate as an alignment mark. The advantage of this method step is that both the structures formed on the substrate front side and the structures formed on the substrate rear side can be aligned with the same structure. As a result, it is possible to avoid a misalignment between the process and exposure steps carried out on the substrate front side and the process and exposure steps carried out on the substrate rear side.
A further method step includes using the structure as an etching mask while etching the rear side. The use of the structure as an etching mask advantageously makes it possible to carry out a self-aligned etching process on the substrate rear side.
One method variant includes forming the structure as a trench in the front side of the substrate. The advantage of this configuration is that a trench in a substrate surface can be used as an alignment mark by a stepper.
A further method variant includes forming the structure as an elevation on the front side of the substrate. An elevation on the substrate front side is suitable for serving as an alignment mark for a stepper. In this case, the elevation can be formed from the same material as the substrate or else from a different material.
If the structure is formed from a different material than the substrate, then the different material can be sunk in the substrate front side and the substrate surface can be planarized.
Furthermore, a doped layer can be formed in the substrate. In this case, the doped layer has the task of serving as an etching stop during the etching of the substrate rear side. The doped layer is advantageously formed in such a way that it acquires the topographic contour of the structure. As a result, it is possible for both the processing on the substrate front side and the processing on the substrate rear side to be aligned with the same structure.
Furthermore, the rear side of the substrate can be etched chemically, during which the substrate is thinned, the doped layer is used as an etching stop, and the structure is formed on the rear side.
In a further method variant, in addition to the structure that is uncovered on the rear side a second trench is formed, which is filled with a mask material. The advantage of this method step is that a mask material can be arranged in a self-aligned manner with respect to the structure formed on the rear side without additional photolithographic exposure steps. The mask material can be used as an etching mask in a subsequent etching step.
In a further method step, the second trench is filled with the mask material by depositing the mask material on the rear side of the substrate and then planarizing. This method step has the advantage that the mask material is introduced into the second trench and forms a self-aligned etching mask, without requiring an additional lithographic exposure step.
A further method step includes using the mask material as an etching mask during the patterning of the rear side. This process step makes it possible, by way of example, to etch contact holes from the substrate rear side to the substrate front side, which are aligned there with corresponding contact areas.
The structure is formed from a mask material and is used as an etching mask for patterning the rear sidexe2x80x94a third trench being formed in addition to the structure. In this method step, the structure on the front side of the substrate is formed from a mask material. The structure, and accordingly, the mask material is uncovered during the etching of the substrate rear side. In a further substrate rear-side etching step, the structure is then used as an etching mask, and a trench is formed in addition to the structure. The advantage of this procedure is that a subsequent lithographic step on the substrate rear side can be aligned with the structure.
A further method step includes filling the third trench with a second mask material and removing the structure from the rear side. The advantage of this method step is that a second mask is formed in addition to the structure. The second mask can then be used as an etching mask in a subsequent etching step in order to etch that region of the substrate rear side that was formerly occupied by the structure, for example, to etch a contact hole as far as the front side of the substrate.
A further advantageous configuration of the method includes filling the third trench with a second mask material and removing the structure from the rear side. This procedure has the advantage that the rear side originally occupied by the structure can be patterned by an etching process in order to form, by way of example, a contact hole to the front side of the substrate.
Furthermore, the second mask material in the third trench can be used as an etching mask during the patterning of the rear side. The use of the second mask material as an etching mask likewise enables an etching process that is self-aligned with respect to the structure, since the third trench and thus the second mask material were formed in a self-aligned manner with respect to the structure.
A further method step includes growing the doped layer on the front side of the substrate. The growth of the doped layer has the advantage that the structure on the front side of the substrate is encapsulated conformally with the doped layer, so that the doped layer has a topography corresponding to the structure.
A further method variant includes forming the first doped layer by implanting dopant in the substrate. The implantation of dopant has the advantage that the doped layer can be introduced conformally with respect to the structure.
A further method variant includes growing a useful layer on the doped layer. The useful layer can be used for example for the formation of circuits or structures.
Furthermore, a circuit can be formed on the front side of the substrate.
One method variant includes growing the useful layer on the front side of the substrate. The useful layer is grown on the structure with a different growth rate or morphology than on the substrate surface. The advantage of this procedure is that a structure formed as an elevation is formed as a depression on the top side of the useful layer.
A further method step includes removing the structure from the front side of the substrate, as a result of which the doped layer is patterned. If the structure is formed as an elevation, for example, and the doped layer is subsequently formed, then by removing the elevation it is possible for the doped layer to be concomitantly removed in the region of the elevation. As a result, it is possible, for example, to form a window in the doped layer.
A further method variant includes etching the rear side of the substrate and using the patterned doped layer as an etching stopxe2x80x94with a fourth trench being formed in the rear side of the substrate in a region in which the doped layer has a mask window. The advantage of this method variant is that the patterned doped layer has a mask window in which a trench can be formed by using a suitable etching step.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for aligning structures on a semiconductor substrate, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.